Potassium channels represent a complex class of voltage-gated ion channels from both functional and structural standpoints. Their functions include regulating neurotransmitter release, heart rate, insulin secretion, neuronal excitability, epithelial electrolyte transport, smooth muscle contraction, and cell volume. In general, four sequence-related potassium channel genes—shaker, shaw, shab, and shal—have been identified in Drosophila, and each has been shown to have human homolog(s). KCNA3 encodes the voltage-gated KV1.3 potassium channel, which is shaker-related and is expressed in lymphocytes (T and B lymphocytes), the central nervous system, fat and other tissues. The functional channel is composed of four identical KV1.3 α-sub units. The KV1.3 potassium channel regulates membrane potential and thereby indirectly influences calcium signaling in human effector-memory T cells (Grissmer S. et al, Proc. Natl. Acad. Sci. U.S.A. 87(23): 9411-5; DeCoursey T. E. et al, Nature 307 (5950): 465-8; Chandy K. G. et al, Trends Pharmacol. Sci. 25(5): 280-9; Wulff H. et al, J. Clin. Invest. 111 (11): 1703-13). Effector memory T cells are important mediators of multiple sclerosis, Type I diabetes mellitus, psoriasis, and rheumatoid arthritis.
The Kv1.3 channel is expressed in T and B lymphocytes in a distinct pattern that depends on the state of lymphocyte activation and differentiation. Upon activation, naive and central memory T cells increase expression of the KCa3.1 channel per cell, while effector-memory T cells increase expression of the KV1.3 channel. Amongst human B cells, naive and early memory B cells express small numbers of KV1.3 and KCa3.1 channels when they are quiescent, and augment KCa3.1 expression after activation. In contrast, class-switched memory B cells express high numbers of KV1.3 channels per cell (about 1500/cell) and this number increases after activation (Chandy K. G. et al, Trends Pharmacol. Sci. 25(5): 280-9; Wulff H. et al, J. Clin. Invest. III (11): 1703-13; Wulff H. et al, J. Immunol. 173(2): 776-86). The Kv1.3 channel promotes the calcium homeostasis required for T-cell receptor-mediated cell activation, gene transcription, and proliferation (Panyi, G et al (2004) Trends Immunol 25:565-569). Kv1.3 is physically coupled through a series of adaptor proteins to the T-cell receptor signaling complex and it traffics to the immunological synapse during antigen presentation. However, blockade of the channel does not prevent immune synapse formation (Panyi G. et al, Proc. Natl. Acad. Sci. U.S.A., 101(5):1285-90; Beeton C. et al, Proc. Natl. Acad. Sci. U.S.A., 103(46): 17414-9). KV1.3 and KCa3.1 regulate membrane potential and calcium signaling of T cells. Calcium entry through the CRAC channel is promoted by potassium efflux through the Kv1.3 and KCa3.1 potassium channels. Blockade of KV1.3 channels in effector-memory T cells suppresses activities like calcium signaling, cytokine production (interferon-gamma, interleukin 2) and cell proliferation. Effector-memory T cells (TEM) were originally defined by their expression of cell surface markers, and can enter sites of inflammation in non-lymphoid tissues, while not participating in the process of lymphoid recirculation carried out by most other lymphocytes. TEMs have been shown to uniquely express high numbers of the KV1.3 potassium channel and depend on these channels for their function. In vivo, KV1.3 blockers paralyze effector-memory T cells at the sites of inflammation and prevent their reactivation in inflamed tissues. In contrast, KV1.3 blockers do not affect the homing to and motility within lymph nodes of naive and central memory T cells, most likely because these cells express the KCa3.1 channel and are therefore protected from the effect of KV1.3 blockade. Suppressing the function of these cells by selectively blocking the KV1.3 channel offers the potential for highly effective therapy of autoimmune diseases with minimal effects on either beneficial immune responses or other organs (Chandy K. G. et al, Trends Pharmacol. Sci. 25(5): 280-9; Wulff H. et al, J. Clin. Invest. III (11): 1703-13; Beeton C. et al, Proc. Natl. Acad. Sci. U.S.A., 103(46): 17414-9; Matheu M. P. et al, Immunity 29(4): 602-14). Kv1.3 has been reported to be expressed in the inner mitochondrial membrane in lymphocytes. The apoptotic protein Bax has been suggested to insert into the outer membrane of the mitochondria and occlude the pore of KV1.3 via a lysine residue. Thus, KV1.3 blockade may contribute to apoptosis (Szabo I. et al, J. Biol. Chem. 280(13): 12790-8; Szabo I. et al., Proc. Natl. Acad. Sci. U.S.A. 105(39): 14861-6).
Autoimmune Disease is a family of disorders resulting from tissue damage caused by a malfunctioning immune system, affecting tens of millions of people worldwide. Such diseases may be restricted to a single organ, as e.g. in multiple sclerosis and Type I diabetes mellitus, or may involve multiple organs as in the case of rheumatoid arthritis and systemic lupus erythematosus. Treatment is generally palliative and typically includes anti-inflammatory and immunosuppressive drugs. The severe side effects of many of these therapies have fueled a continuing search for more effective and selective immunosuppressive drugs. Among these are those which can selectively inhibit the function of effector-memory T cells, known to be involved in the etiology of many of these autoimmune diseases and thereby ameliorate many autoimmune diseases without compromising the protective immune response. Multiple sclerosis is a disease caused by autoimmune damage to the central nervous system including the brain, which affects roughly two and a half million people worldwide. Symptoms include muscle weakness and paralysis, and the disease can progress rapidly and unpredictably and may eventually lead to death. Treatment usually includes the use of anti-inflammatory and immunosuppressive drugs which have potentially severe side effects. KV1.3 has been shown to be highly expressed in autoreactive effector memory T cells from MS patients (Wulff, H et al (2003) J Clin Invest 111:1703-1713; Rus H et al (2005) PNAS 102:11094-11099). Animal models of multiple sclerosis have been successfully treated using blockers of the KV1.3 potassium channel. In patients with multiple sclerosis, disease-associated myelin-specific T cells from the blood are predominantly co-stimulation independent effector-memory T cells that express high numbers of KV1.3 channels. T cells in MS lesions in postmortem brain lesions are also predominantly effector-memory T cells that express high levels of the KV1.3 channel (Wulff H. et al, J. Clin. Invest. 111(11): 1703-13; Beeton C. et al, Proc. Natl. Acad. Sci. U.S.A. 103(46): 17414-9).
Type 1 diabetes mellitus is a disease caused by autoimmune destruction of insulin-producing cells in the pancreas, resulting in high blood sugar and other metabolic abnormalities. Type 1 diabetes mellitus affects close to four hundred thousand people in the US alone, and is usually diagnosed before age 20. Its long-term consequences may include blindness, nerve damage and kidney failure, and left untreated is rapidly fatal. Treatment involves life-long administration of insulin or pancreas transplantation, both of which may entail serious side effects (Beeton C. et al, Proc. Natl. Acad. Sci. U.S.A. 103(46): 17414-9).
Kv1.3 is also considered a therapeutic target for the treatment of obesity, for enhancing peripheral insulin sensitivity in patients with type-2 diabetes mellitus, for preventing bone resorption in periodontal disease, for rheumatoid arthritis, for inflammatory skin conditions, such as psoriasis, and for asthma (Tucker K. et al, Int. J. Obes. (Lond) 32(8): 1222-32; Xu J. et al, Hum. Mol Genet. 12(5): 551-9; Xu J. et al, Proc. Natl. Acad. Sci. U.S.A. 101(9): 3112-7; Valverde P. et al, J. Dent. Res 84(6): 488-99; Tschritter O. et al, J. Clin. Endocrinol. Metab. 91(2): 654-8; Beeton, C. et al, Proc. Natl. Acad. Sci. U.S.A. 103(46): 17414-17419; Azam, P. et al, J. Invest. Derm. 127: 1419-1429; Bradding, P et al, Br. J. Pharmacol. 157: 1330-1339).
Compounds which are selective KV1.3 blockers are thus potential therapeutic agents as immunosuppressants or immune system modulators including for the prevention of graft rejection, and the treatment of autoimmune and inflammatory disorders. KV1.3 modulators may be used alone or in conjunction with other immunosuppressants, such as selective KCa3.1 blockers or cyclosporin, in order to possibly achieve synergism and/or to reduce toxicity, especially of cyclosporin. At present there exist a number of non-selective K channels that will inhibit lymphocyte proliferation, but have adverse side effects. Other K channels exist in a wide range of tissues including the heart and brain, and generally blocking these channels is undesirable. U.S. Pat. No. 5,494,895 discloses the use of a thirty-one amino acid peptide, scorpion peptide margatoxin, as a selective inhibitor and probe of KV1.3 channels present in human lymphocytes, and also as an immunosuppressant. However the use of this compound is limited by its potent toxicity.
International patent Application publications numbers WO 97/16438 and WO 09/716437, and U.S. Pat. No. 6,051,590 describe the use of the triterpene, correolide and related compounds as immunosuppressants in the treatment of conditions in mammals affected or facilitated by KV1.3 inhibition.
There is still a need for improved and specific therapies for immune diseases, including autoimmune diseases, and for immunosuppressive agents which lack problematic side effects and specifically target channels involved in immune cell mediated actions.